Cerebellar Abiotrophy: Not Just for Arabians?

by: Erica Larson, News Editor

May 10 2011Article # 18228

A severe, incurable neurologic disorder thought to affect primarily Arabians--cerebellar abiotrophy (CA)--might be able to jump the "breed barrier" and affect other types of horses, according to a University of California, Davis, research team. The genetic disease--caused by a mutation on a certain segment of DNA--other breeds of horses (mostly those with Arabians in their lineage) could be at risk for carrying the CA mutation.

Cerebellar abiotrophy is caused by the loss of a specific type of neuron in the cerebellum (the part of the brain that controls a horse's sensory perception, coordination, and motor control). As a result, affected foals suffer from head tremors and a lack of equilibrium, among other neurologic deficits. Due to the absence of an effective treatment approach the more severely affected foals are routinely euthanized early in life because of the risk they would pose to themselves and others as adults.

Cecilia Penedo, PhD, of the UC Davis Veterinary Genetics Laboratory (VGL), along with Leah Brault, PhD, who worked on the continuing CA project at UC Davis during her doctoral studies, identified a genetic mutation associated with CA in 2010. But they began to wonder if the Arabian's role in the development of many other breeds could mean that these breeds could have inherited the CA mutation.

Brault and Penedo tested archived DNA samples at the VGL from 1,845 horses of 31 different breeds (not including Arabians) to determine if the CA mutation was present in any other breeds. At least 40 horses from each breed were tested. Interestingly, the study found that six of 1,845 non-Arabian horses carried the CA mutation. The affected horses included a Welsh Pony, two Trakehners, and three Bashkir Curlies. The team noted that all of these horses were heterozygous (one mutated gene in a pair) rather than homozygous (two mutated genes in a pair), meaning the horses carried CA but were not clinically affected by the disorder.

The team furthered their findings by examining the CA carriers' pedigrees. They found that one of the carrier Trakehners was ¾ Arabian, and the other Trakehner and the Welsh Pony each had one Arabian parent. "The Arabian horses used in these crosses traced their pedigrees back to horses found on both sides of the pedigrees of affected Arabian horses," Penedo noted.

According to Brault and Penedo, the CA mutation found in the Bashkir Curlies likely originated from one Arabian stallion used to develop the breed in the 1960s--a stallion present in many Curly horses' pedigrees today.

Additionally, the researchers tested 92 Trakehners, 88 Bashkir Curlies, and 86 Welsh ponies' DNA to determine the approximate frequency of the CA mutation in each breed:

No additional carriers were identified among the Trakehners or the Welsh ponies, bringing the estimated CA-carrier frequency to 1.40% and 0.70% respectively; and

Five additional Bashkir Curlies were identified as carriers (all of which traced their lineage to the same stallion noted previously), increasing the estimated frequency to 5.6%.

So what does this mean for horse owners?

Especially for breeds with Arabian influence, the researchers suggest that it might be "beneficial to utilize genetic testing of breeding stock for the CA mutation, so that breeders can make informed choices about the animals they choose to mate.

"At present, the random chance of breeding two carriers is low, about 0.3% in Bashkir Curly, 0.02% in Trakehner, and 0.005% in Welsh Pony," Penedo said. "If two carriers were bred, 25% of foals would be expected to be CA-affected. Thus, a very small risk.

"However, matings are not random, and there is the potential that frequency of carriers can be amplified by preferential use of a carrier stallion (i.e., breeding to a popular sire or founder effect)," she continued. "The consequence of this is that, with time, the incidence of carriers could increase in the breed and so would the risk of producing CA-affected foals."

The good news is that a number of "heavily Arabian-influenced breeds," such as the Thoroughbred and Saddlebred, were tested for the CA mutation in the study and no carriers were found, "suggesting that they have escaped the introduction of the CA mutation from Arabian introgression."

"Breeds that allow registration of horses with Arabian ancestry should have any Arabian breeding stock tested for the mutation, and breeds descended from Arabian ancestry should pursue genetic testing of breeding stock to prevent the occurrence of affected foals," Brault and Penedo concluded.

The study, "The frequency of the equine cerebellar abiotrophy mutation in non-Arabian horse breeds," has been accepted for publication in an upcoming issue of Equine Veterinary Journal. The abstract is available online.